1. Field of the Invention
The present invention relates to a camera with a function of preventing a hand moving blur. More particularly, the invention relates to a camera with a function of preventing a hand moving blur of a picture caused by movement of the camera which the function appropriately and reliably prevents such blur on the condition that the camera sets the shutter speed which is as close as possible to that which the user wants.
Note that the term "shutter speed" means the time period when the shutter is being opened.
2. Description of the Related Art
One of the techniques to prevent blurring of the picture caused by movement of the camera is to control the starting point of exposure (timing of exposure). Such a hand movement blur prevention mechanism is disclosed in, for example, Japanese Patent Application Laying Open (KOKAI) No. 63-53531.
The proposed technique disclosed in the publication is to prevent the blurring of images on the film surface by such a way that the speed of movement of the camera is detected to determine a peak value of the movement from the detection signal and that the exposure is started at the time around the timing of the peak of the movement.
On the other hand, Japanese Patent Application Laying Open (KOKAI) No. 53526 discloses another technique to prevent blurring of a picture caused by hand movement.
The second technique is to prevent the blurring of the picture on the condition that the picture is taken at a shutter timing which is set to be as close as possible to that which the user intends to set.
More precisely, according to the second technique, the blurring is prevented by such a way that the movement amount of the camera at the time of releasing the shutter is detected with the use of a plurality of acceleration sensors that the camera is equipped with and that a shutter speed which is as close as possible to that which the user wants is set on the basis of the detected movement amount of the camera.
That is, first, four acceleration sensors are disposed at two vertically separated two points and two horizontally separated points in the camera, respectively, each sensitive axis of the sensor being parallel to the optical axis of the camera lens or perpendicular to the optical axis.
On the other hand, the measurement reference time necessary for detecting the movement amount of the camera is set immediately before releasing the shutter. The reference time is divided into a plurality of detection time periods in series according to the time unit corresponding to a desired shutter speed so that the movement amount of the camera is detected in each of the detection time periods.
After that, the difference between the maximum peak of the camera movement amount and the minimum peak amount thereof is calculated. The calculation result is used as the probability base to determine the presumptive speed of the image movement on the film surface.
On the basis of the presumptive speed of the image movement, the shutter speed is determined by trial and error, shutter speed being as close as possible to the desired speed and at the shutter speed the image movement speed on the film surface being within the allowable range.
It is desirable that the starting timing of exposure be set so that the peak point of the hand movement is coincident with the central point of the exposure time period. Under this condition, it is proposed to prevent blurring caused by hand movement by controlling the starting point of exposure at the desired shutter speed.
For example, above-mentioned Japanese Patent Application Laying Open (KOKAI) No. 63-53531 discloses a technique in which a differential circuit or an integration circuit is used in conjuction with a phase shift filter to realize the above-mentioned condition.
However, since the speed of the camera movement and the corresponding shutter speed vary, the technique of the publication mentioned above can not completely prevent blurring in some conditions of the speed of the camera movement.
On the other hand, compact cameras equipped with a large magnification lens having a focal length of more than 100 mm are widely used. When taking a picture using a large magnification lens, if the user is inexperienced, the picture is often blurred due to the hand movement at the shutter speed of 1/30 to 1/125 sec. since the F-number of the lens becomes large, i.e., the lens becomes dark.
Therefore, it is required to realize a means for reliably preventing the blurring of the picture caused by hand movement at a shutter speed of 1/30 to 1/125 in particular.
The inventors of the present invention found out through experiments that the blurring of image on the film surface occurs incidentally and the extent of that is not constant.
The inventors also found out that when a picture is taken by a camera held by hands, the image on the film surface vibrates at a low frequency less than 2 Hz or a frequency of about 10 Hz with amplitude being not constant.
The inventors further confirmed that the blurring appearing on the picture was mainly caused by the vibration of 10 Hz due to the tremble of fingers at the time of releasing the shutter.
Therefore, it is necessary to suppress the image movement speed due to the vibration of 10 Hz so that the speed is kept within an allowable range.
This means that the blurring of the picture caused by hand movement can not essentially be avoided by the technique of the prior art mentioned above in which the presumptive speed of image on the film surface is determined from the hand movement speed at the time immediately before exposure so as to control and to minimize the blurring on the basis of the presumptive speed.
Also, with regard to the structure comprising a differential circuit or an integration circuit in conjuction with a phase shift filter as disclosed in Japanese Patent Application Laying Open (KOKAI) No. 63-53531, the construction of the camera becomes complicated and disadvantageous to realize a compact multi-functional camera.
Therefore, it is required to realize a camera which is simple in construction and equipped with a mechanism which reliably prevents the blurring of picture caused by hand movement.